When plate-like foams of synthetic resins are used as cushioning materials for cushioning packages, there are two principles for achieving the cushioning performances thereof; namely, one of which is based on the buckling destruction of a foam, and the other of which is based on the elastic deformation of a foam, thereby absorbing impact stress to achieve the cushioning ability.
Since said principles utilize the foam properties that are contrary to each other, they have advantages and disadvantages that are contrary to each other, having their own technical fields for cushioning design, and are selectively used depending on what contents are to be cushioned.
Specifically, in the former case where rigid forms typified, for example, by polystyrene resin foams, there is an advantage such that a large load (i.e., a large stress) can be cushioned with relatively smaller supporting area (i.e., with a foam of a small volume), but, on the other hand, there is involved a disadvantage such that the foam cannot endure repeated impacts, or a space formed between contents and foams (i.e. cushioning materials) subjected to buckling deformation may cause "load dislocation", whereby the contents are frictionally damaged. Accordingly, such foams are frequently used for cushioning packages placed under good control during the course of the freight handling, and containing middle class heavy goods having the surface of high hardness.
In contrast, foams having the closed-cell structure, typically foams of polyethylene resins, are suited for use in the field utilizing flexibility and the repeated elastic recovery inherent in the foams of this sort, but are not suitable for cushioning of a large load. If, for example, the density of foams is increased in order to cope with such a difficulty, flexibility will be lost, and moreover, if they are used with increased supporting areas, the packaging style must be made to be of a large scale, resulting in losing economical merits in the various phases accompanied therewith to a great extent. Accordingly, foams of this sort are frequently used for cushioning packages in which only a high cushioning performance for light weight contents is required.
In designing the cushioning for such cushioning packages, improvement of foams which are cushioning materials has been sought after.
For example, if, in the field of the soft foams in which the above elastic deformation is desirably utilized, it be possible to provide foams having higher compression, strength (i.e., compression stress) than the conventional polyethylene foams, while sufficiently keeping the flexible recovery property (i.e., a low density or highly foamed product), it would be expected to widen the field of utilization of elastic cushioning for a larger load (i.e., heavier contents). Accordingly, to accomplish such foams, developments have been made for a long time for foams of resin compositions comprising a mixture of polyolefin resin and polystyrene resin, and also a number of means for mixing them has been proposed.
Since both of these resins are poor in compatibility with each other, it was impossible to obtain foams having good quality. Accordingly, there have been a large number of proposals for improvement of mixing conditions or improvement in how to copolymerize them in order to attain homogeneous dispersion of them.
Of the proposals, what should be noted is a technique disclosed in the specification of European Patent No. 55,460. This discusses how to physically disperse polyolefin resins and polystyrene resins, and mainly discloses a production method showing a mixing condition in which the character of combination of the resins to be mixed is governed by the relation between the ratio (R) of mutual rheological characteristics and the proportion (Y) of the content of styrene resins, and the relation between the density (D) of resultant foams and strength (F) under application of 25% compression.
The invention disclosed in the above patent has been assigned to the applicant of the present invention, and in part has been put into practical use and the product thereof has been made commercially available with a trademark "SUNTECK FOAM".
The commercially available product disclosed in the European patent has good performances as described in its specification, and has attracted attention as a product which has satisfied its target for the time being.
However, when it was put into a full-scale practical use, it was found that there were unexpected practical problems remained unsolved in the foams of resin mixtures obtained by the method of the above European patent.
That is, of the main problems, the first is a problem such that foams are subject to cracking due to repeated drop impacts (in particular, drop-on-corner) during transportation and no desired cushioning coefficient can be exhibited, the second is a problem such that the foams are subject to fatigue deformation due to repeated vibration impacts during transportation to cause "load dislocation", the third is a problem such that, when foams are die-cut to make cushioning materials, the surfaces thereof are cracked and destroyed along the outline of die cutting to lose the quality needed as a pad, and further the fourth is a problem such that, when foams are used as cushioning materials, foams are adhered to corrugated fiberboards in order to fix the cushioning materials to predetermined positions in a package, but the adhered foams are liable to peel therefrom with the result that no cushioning design value designed as a package is attained.
Accordingly, the foams according to the production method in said European patent has not been well accepted by the industry because they involved many restrictions in the manner of application.
The present inventors have presumed that these problems may have originated from the mutual incompatibility of the two resins, giving no fitness between the interfaces of the resins even if they may appear to have been integrated together, and accordingly a third substance would be necessarily required to be present in order to improve the mix of polyolefin resins and polystyrene resins. And, as a result of surveys and researches, they have taken a note of a technique disclosed in the U.S. Pat. No. 4,020,025; namely, a technical concept such that foams of resin mixtures of both of polyolefin resins and polystyrene resins are obtained in the presence of a hydrogenated styrene/butadiene block copolymer.
The reason therefor is as follows: The foams themselves disclosed in said U.S. patent involve insufficiencies from a practical viewpoint of properties when considering our aim to provide cushioning materials for use in the field of elastic cushioning under a large load (i.e., a high stress), namely, they have a number of drawbacks such that;
(a) cushioning materials are subject to deformation (by fatigue) due to repeated vibration impacts during transportation to cause the load dislocation;
(b) foams are subject to cracking due to repeated drop impacts;
(c) they are not suitable for use as cushioning materials in the direction to which a torsional stress or a shear stress is applied to the foam;
(d) they are subject to large creep to cause the load dislocation;
(e) when foams are die-cut, the surface portions are destroyed along the outline of die-cutting;
(f) they have poor adhesiveness to a corrugated fiberboard; and
(g) when large compression is applied foams can not retain the original properties (such as strength).
Also, the level of quality possessed by them is not superior, or even inferior, to the foams disclosed in the above European patent. However, on the other hand, when the state of dispersion of polystyrene resins in the polyolefin resins was compared with that in the method of said European patent, there was a feel of the goodness in character as a material different from that of the foams obtained according to the method of said European patent or a possibility of achieving an improvement through a different route. Thus, it was decided to cope with the improvement of the foams disclosed in said U.S. patent.